Osteoporosis and subsequent fractures are an important health problem facing the elderly. Four major risk factors for osteoporosis are age, gender, genetic predisposition, and obesity. Here we propose to examine dietary and genetic factors affecting bone mass, morphology, and biomechanical properties and the relationships of these features to obesity in an established mouse model for obesity, diabetes, and dietary response, the cross of LG/J and SM/J mouse strains. Through these studies we will discover novel genes and pathways affecting bone and its relationship to obesity. We will measure the level of heritability for bone traits and their genetic correlations with obesity and leptin levels in the LGXSM Recombinant Inbred (Rl) strains. Animals from each strain have been fed a high or low fat diet allowing us to examine the effects of both genes and environment (dietary fat) on bone characteristics and bone-obesity relationships. We will identify genomic regions (Quantitative Trait Loci, QTLs) affecting bone and its relationship to obesity in the Rl strains, F2 intercross, and an F10 Advanced Intercross (Al) line population. All animals were reared and genotyped for previous projects and are available to this project at no additional cost. We will also fine-map these QTLs in the F32 generation of the Al Line (N = 1000) to validate and fine-map the QTLs discovered with earlier populations. Unlike the F2 and F10 generations, complete obesity, leptin, and diabetes-related phenotypes will be collected in the F32 animals and animals will be reared on either a high or low fat diet. Fine-mapping in this generation will allow us to identify a small set of positional candidate genes for for further evaluation. Sequence and expression polymorphism will be examined for these genes. Strongly supported candidate genes will be evaluated in knock-out and overexpressing transgenic mice. This project will identify novel genes and physiological pathways affecting osteoporosis and examine how genetic and environmentally-based obesity affects bone mass, morphology and biomechanical function.